This invention relates to ultrasonic transducers and, more particularly, to ultrasonic transducers for the detection of vibrational resonances of an object.
Conventional ultrasonic transducers may be used to obtain the resonant frequency spectrum of a component in response to the excitation of the component by ultrasonic sound waves, i.e., sound waves having frequencies generally above about 20 kHz. Such transducers may be "bare" transducers, i.e., bare piezoelectric crystals with electrical leads attached, or commercial transducer assemblies with component parts that are adhesively bonded together.
A problem with conventional transducers is that the measured response spectrum is composed of resonances from both the object being tested and the transducer. As illustrated in FIGS. 3A-3C, even simple bare transducers have natural resonances and harmonics that start at low frequencies and extend up to several megahertz. A system with either a noisy driver (exciter) or detector or both will have peaks in the resonant ultrasound spectrum (RUS) that originate from the noise source. The analysis of the RUS must account for these noise peaks. However, it is not possible to define the position of these noise peaks a priori because any object used for the calibration will itself have resonances that will be in the spectrum. Transducers formed from adhesively bonded components will have additional resonance peaks arising from the compliance of the adhesive bonding.
It would be desirable to provide an ultrasonic transducer that does not introduce self-resonances into the RUS from an object being analyzed. Accordingly, it is an object of the present invention to provide a bonded ultrasonic transducer with highly reproducible properties and without self-resonant responses in a RUS range of interest.
It is another object of the present invention to make robust transducers for use in non-destructive testing applications to output response signals that accurately represent the resonant response of an object.
Yet another object of the present invention is to provide a method for fabricating ultrasonic transducers that have a broadband linear response.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.